Journal of Personalized Medicine Review The Emerging Role of CD24 in Cancer Theranostics—A Novel Target for Fluorescence Image-Guided Surgery in Ovarian Cancer and Beyond Katrin Kleinmanns 1,* , Vibeke Fosse 1, Line Bjørge 1,2 and Emmet McCormack 1,3,4 1 Center for Cancer Biomarkers CCBIO, Department of Clinical Science, University of Bergen, Jonas Lies vei 91B, 5021 Bergen, Norway; [email protected] (V.F.); [email protected] (L.B.); [email protected] (E.M.) 2 Department of Obstetrics and Gyneacology, Haukeland University Hospital, 5021 Bergen, Norway 3 Centre for Pharmacy, Department of Clinical Science, University of Bergen, Jonas Lies vei 65, 5021 Bergen, Norway 4 Department of Clinical Science, University of Bergen, Jonas Lies vei 65, 5021 Bergen, Norway * Correspondence: [email protected] Received: 10 November 2020; Accepted: 26 November 2020; Published: 27 November 2020 Abstract: Complete cytoreductive surgery is the cornerstone of the treatment of epithelial ovarian cancer (EOC). The application of fluorescence image-guided surgery (FIGS) allows for the increased intraoperative visualization and delineation of malignant lesions by using fluorescently labeled targeting biomarkers, thereby improving intraoperative guidance. CD24, a small glycophosphatidylinositol-anchored cell surface receptor, is overexpressed in approximately 70% of solid cancers, and has been proposed as a prognostic and therapeutic tumor-specific biomarker for EOC. Recently, preclinical studies have demonstrated the benefit of CD24-targeted contrast agents for non-invasive fluorescence imaging, as well as improved tumor resection by employing CD24-targeted FIGS in orthotopic patient-derived xenograft models of EOC. The successful detection of miniscule metastases denotes CD24 as a promising biomarker for the application of fluorescence-guided surgery in EOC patients. The aim of this review is to present the clinical and preclinically evaluated biomarkers for ovarian cancer FIGS, highlight the strengths of CD24, and propose a future bimodal approach combining CD24-targeted fluorescence imaging with radionuclide detection and targeted therapy. Keywords: biomarker; intraoperative imaging; debulking surgery; complete resection; epithelial ovarian cancer; optical imaging; CD24 1. Introduction The aggressive surgical approach used to treat metastatic ovarian cancer is unique and remains one of the therapeutic keystones, with the aim of achieving curative treatment. The overall survival rate for epithelial ovarian cancer (EOC) is below 50%, mainly because the lack of early symptoms leads to a late diagnosis in almost 75% of patients, followed by limited treatment options and emerging drug resistance [1]. In 1975, Thomas Griffith was the first to show that besides the histological subtype, the extent of residual disease after surgery impacts patient outcome [2]. The extent of residual disease is classified into complete cytoreduction (0 cm) and macroscopic residual disease, which is further subclassified into optimal (<1 cm) and suboptimal (>1 cm) cytoreduction [3]. The impact of complete debulking has been confirmed in many trials as the most important prognostic factor for the survival of EOC patients, and it has been suggested to increase the efficacy of subsequent drug therapies [3–5]. Clinical trials have also indicated that if macroscopic residual disease remains after surgery, the patient J. Pers. Med. 2020, 10, 255; doi:10.3390/jpm10040255 www.mdpi.com/journal/jpm J. Pers. Med. 2020, 10, 255 2 of 18 has no significant benefit compared to those who received suboptimal debulking, highlighting the need for better tools to help surgeons achieve complete resection. In clinical reality, complete debulking is not always feasible and can be challenging because of aggressive tumor biology, histological subtype, advanced disease stage, and unresectable tumors near vital structures [6–8]. To detect submillimeter and residual tumor lesions, the surgeon relies on tactile and visual inspection, training, and experience. New technologies for preoperative planning are warranted to help identify patients who will benefit from surgery, and for these patients, intraoperative guidance methods to help achieve complete debulking are crucial for their survival. One surgical technology that was developed to meet these criteria is fluorescence image-guided surgery (FIGS). FIGS employs fluorescent molecules (fluorophores) as contrast agents, and together with specialized imaging systems, the surgeon is provided with a screen image highlighting fluorescent tumor tissue in the operating field. This technology has the capability to provide the surgeon with real-time feedback and optimize the precision of resection, thus improving clinical outcomes. FIGS is currently being assessed in ovarian cancer clinical trials, as well as for many other solid malignancies. Ovarian cancer Phase I–III FIGS trials are being performed with untargeted dyes, such as indocyanine green (ICG) [9] and contrast agents that target the folate receptor alpha (FRα). The first in-human FIGS study was performed by Go van Dam et al. in 2011, in which they demonstrated the feasibility of FIGS to improve the visualization of malignant lesions with a tumor-specific FRα-targeting probe, which had high specificity for peritoneal metastases and left cancer-free tissue fluorescence negative [10]. In fact, this tumor-specific fluorescence approach resulted in an increased resection rate for malignant tissue. Since this first pilot study, FRα-targeting FIGS contrast agents have been improved, and there are promising results from Phase II [11,12] and ongoing Phase III (NCT03180307) clinical trials. To expand beyond folate and other clinically approved monoclonal antibodies (mAbs), much effort is being dedicated to identifying new biomarkers for FIGS. Notably, the challenge faced when developing a good imaging contrast agent for FIGS is the identification of both a biomarker that can discriminate between malignant and healthy tissues and a dye capable of maximal tissue penetration which can be detected with a dedicated and sensitive fluorescence imaging system. The biomarker-dye conjugate also needs to exhibit low toxicity, high stability, and fast clearance to reduce unspecific signals [13]. Cluster of differentiation 24 (CD24) is a novel molecular target used for imaging, molecular-targeted drug therapy, and immunotherapy [14–16]. Its high expression on tumor cells and rare expression on healthy human tissue, where it is primarily found on hematologic cells, indicate its potential as a tumor-specific biomarker [14–16]. A meta-analysis of 28 studies revealed that CD24 is overexpressed in 68% of human cancers, and CD24 expression was correlated with a higher self-renewal ability, more metastases, and a poor prognosis [17]. In EOC, CD24 is almost uniformly (70.1–100%) expressed [18,19]. We have demonstrated the ability of a CD24-targeting mAb conjugated with near-infrared (NIR) fluorophores to identify tumor lesions in metastatic EOC patient-derived xenograft (PDX) models with heterogeneous CD24 expression, supporting its potential to translate CD24-guidance to the intraoperative setting for EOC patients [14,19]. Here, we review the emerging tumor-specific biomarker CD24 as a target for FIGS, and focus specifically on EOC and the use of CD24 as a bimodal biomarker for both imaging and theranostics. 2. Fluorescence Image-Guided Surgery FIGS is a new strategy with the potential to (1) enhance the visualization of submillimeter metastases, (2) improve tumor staging and risk stratification, (3) increase contrast between malignant and healthy tissues, (4) optimize negative resection margins, (5) minimize damage to healthy structures, and (6) confirm tumor cell-free resection beds intraoperatively in (7) real-time [20]. In a successful debulking surgery with complete removal, all malignant lesions are resected, tumor cell-infiltrated lymph nodes are removed, and negative tumor margins are achieved without damaging vital and healthy structures [20]. In EOC clinical trials focused on the impact of residual disease, complete debulking surgery (0 mm residual disease) was achieved in 33.5–37% of cases, while optimal debulking J. Pers. Med. 2020, 10, 255 3 of 18 surgery was achieved in 64.7–74% of cases [3–5]. In addition, among several different solid cancers, positive tumor margins occur in 8–70% of tumor resections, which correlates with local recurrence and a poor prognosis [13,21,22]. Although FIGS appears to be an ideal emerging technology to help prevent incomplete tumor surgeries, and several imaging probes have been shown to allow differentiation between malignant and healthy tissues, the probes often fail to achieve sufficient contrast to clearly delineate the tumor margins. This highlights the challenges facing the development of sensitive and specific FIGS probes—the identification of compatible and suitable dyes, biomarkers, and imaging technologies that together allow the specific targeting and intraoperative identification of malignant lesions [23]. Dyes currently being investigated in fluorescence optical imaging technologies for clinical use have shifted from those that emit in the visible light spectrum to dyes that emit in the NIR spectrum (700–900 nm) [13,20]. The NIR spectrum offers reduced tissue autofluorescence, scattering, and tissue absorption characteristics, therefore allowing deeper tissue penetration with better contrast compared to the visible light spectrum (400–700 nm). Recent advances in NIR-II (900–1700 nm) fluorescence probes